Estimation of energy expenditure

ABSTRACT

An apparatus ( 1 ) for estimating the energy expenditure of a patient ( 2 ) comprises means ( 3 ) for receiving a set of measurements ( 6 ) from a ventilator ( 4 ), wherein the set of measurements ( 6 ) comprises at least one gas concentration measurement. The apparatus ( 1 ) further comprises means ( 7 ) for estimating the energy expenditure of the patient ( 2 ) based on the set of measurements ( 6 ).

FIELD OF THE INVENTION

The present invention relates to estimating the energy expenditure of apatient. In particular, the invention relates to estimating the energyexpenditure of a patient receiving assisted respiration from aventilator.

BACKGROUND OF THE INVENTION

A mechanical ventilator is an apparatus which provides breathingassistance to a patient who is physically unable to breathesufficiently. Mechanical ventilators are often referred to asrespirators, or simply as ventilators. The ventilator provides breathingassistance by mechanically moving air into and out of the patient'slungs. Modern ventilators tend to be computerised devices and are usedin intensive care medicine, home care, emergency medicine andanaesthesia. Such a ventilator typically comprises a gas reservoir orturbine and air and oxygen supplies. The respiratory gases aretransported between the ventilator and the patient via disposable orreusable pulmonary tubes. The pulmonary tubes are a conduit which is influid communication with both the ventilator and the patient in order totransport the inspired and expired gases therebetween. To be in fluidcommunication with the patient, the pulmonary tube commonly compriseseither a face mask or a tracheal tube. The ventilator may additionallyinclude a humidifier, water traps, a nebulizer, sensors, and variousconnectors and valves. By means of the sensors, the ventilator is ableto monitor certain patient-related parameters (such as pressure, volumeand flow) in order to ensure that the patient is receiving the correctrespiration assistance for his or her physiology.

In an intensive care unit (ICU), information regarding a patient'smetabolism is important for determining the correct amount of clinicalnutrition needed. This is especially important for ICU patients, who aregenerally unable to ingest food, and whose nutritional requirements mustbe met with the highest possible accuracy in order to avoid underfeedingor overfeeding. The energy expenditure of a patient can be estimatedusing existing empirical models, but these estimates may not be accuratein the case of a mechanically ventilated patient, whose condition tendsto change drastically over time.

An indirect calorimeter may be employed to assess a patient's energyexpenditure. An indirect calorimeter measures various properties of theair inhaled and exhaled by the patient in order to estimate the energyexpended by the patient's metabolism. However, it is extremely dangerousto perform such measurements on patients who are receiving respiratoryassistance from an ventilator since they are often in a critical stateof health. Disconnecting such a patient from the ventilator, even for ashort amount of time, to conduct measurements with an indirectcalorimeter would put the patient's life at great risk and is highlyundesirable.

U.S. Pat. No. 5,072,737 describes a method and apparatus for measuringmetabolic rates of a patient intubated on a ventilator. An inspirationsample of gases provided by the ventilator is collected. End-tidal andambient pressure samples of gases exhaled by the patient are alsocollected. A sensor means is provided external to the ventilator and isarranged to receive gas samples via three different conduits. The sensormeans includes an oxygen sensor, a carbon dioxide sensor and a pressuresensor, and provides a signal indicative of an unknown parameter of agas sample. A computer uses information from the various sensors tocompute breath-by-breath flow weighted average rates of oxygenconsumption and carbon dioxide elimination. In other words, according toU.S. Pat. No. 5,072,737, the ventilator is solely used to measure theflow rate of exhaled gas, whilst all the other measurements are taken bymeans of external sensors. Thus, the apparatus of U.S. Pat. No.5,072,737 requires numerous additional conduits and sensors to beconnected to the ventilator.

SUMMARY OF THE INVENTION

It is a preferred aim of the invention to overcome or mitigate theproblems and disadvantages described above.

A first aspect of the invention provides an apparatus for estimatingenergy expenditure of a patient, the apparatus comprising: means forreceiving a set of measurements from a ventilator, wherein the set ofmeasurements comprises at least one gas concentration measurement; andmeans for estimating the energy expenditure of the patient based on theset of measurements. The term “energy expenditure” used herein ispreferably understood to refer to the metabolic energy expenditure ofthe patient.

Hence, the apparatus advantageously uses measurements that are alreadyavailable from a ventilator to estimate the energy expenditure of apatient. This avoids the need for a separate indirect calorimeter, whichwould require periodic and laborious calibration. Furthermore, theapparatus is also simple to use for staff in an intensive care unit,because it makes use of a ventilator with which they are alreadyfamiliar. Yet further, the apparatus avoids the need to insert anexternal device into the pulmonary circuit between the ventilator andthe patient, and thereby reduces the possibility for inaccuracy causedby perturbation of thermodynamic and mechanical parameters that mayresult from inserting an external device into the pulmonary circuit.

The at least one gas concentration measurement preferably comprises anexpiratory carbon dioxide concentration measurement and/or aninspiratory oxygen concentration measurement. The apparatus preferablyfurther comprises means for estimating an inspiratory carbon dioxideconcentration based on the inspiratory oxygen concentration measurement.The apparatus preferably further comprises means for estimating aninspiratory carbon dioxide concentration based on a known concentrationof carbon dioxide in medical air. The means for estimating the energyexpenditure is preferably operable to estimate the energy expenditurebased on an estimate of inspiratory carbon dioxide concentration.Estimating the inspiratory carbon dioxide concentration avoids the needfor a sensor to measure inspiratory carbon dioxide concentration,thereby simplifying the apparatus. Estimating the inspiratory carbondioxide concentration also avoids the difficulties associated withmeasuring the inspiratory carbon dioxide concentration, which is toosmall to measure reliably.

The apparatus preferably further comprises means for receiving anexpiratory oxygen concentration measurement, wherein the means forestimating the energy expenditure is operable to estimate the energyexpenditure based on the set of measurements and the expiratory oxygenconcentration measurement. The apparatus is operable to receive theexpiratory oxygen concentration measurement from a sensor that isseparate from the ventilator. In this context, the term “separate fromthe ventilator” is preferably understood to mean that the sensor is notpart of the ventilator. To put this another way, the sensor ispreferably an additional component that is not supplied with theventilator, but which is added to allow the energy expenditure of thepatient to be estimated. By making use of measurements that are alreadyavailable from a ventilator, the apparatus advantageously requires onlyone additional sensor to measure expiratory oxygen concentration. Thissimplifies the construction and reduces the cost of the apparatus.Preferably, the sensor for measuring oxygen concentration is located ina pulmonary tube, in close proximity to the patient. The close proximityof the sensor to the patient improves the accuracy of the expiratoryoxygen concentration measurement. Alternatively, the apparatus could beoperable to receive the expiratory oxygen concentration measurement fromthe ventilator itself; in this case, the set of measurements from theventilator would further comprise an expiratory oxygen concentrationmeasurement.

The set of measurements preferably further comprises an expiratoryvolume measurement and/or an inspiratory volume measurement. The set ofmeasurements more preferably comprises only one of an expiratory volumemeasurement and an inspiratory volume measurement. The apparatuspreferably further comprises means for estimating an inspiratory volumebased on the set of measurements. More specifically, the apparatus canestimate the inspiratory volume based on an expiratory volumemeasurement, an inspiratory oxygen concentration measurement, anexpiratory carbon dioxide concentration measurement and an expiratoryoxygen concentration measurement. Alternatively, the apparatuspreferably further comprises means for estimating an expiratory volumebased on the set of measurements. More specifically, the apparatus canestimate the expiratory volume based on an inspiratory volumemeasurement, an inspiratory oxygen concentration measurement, anexpiratory carbon dioxide concentration measurement and an expiratoryoxygen concentration measurement. Estimating the inspiratory volume orexpiratory volume improves the accuracy of the estimate of the patient'senergy expenditure. Estimating one of the inspiratory volume orexpiratory volume can also avoid the need for a sensor to measure theother volume, thereby simplifying the apparatus. Preferably, theestimate of inspiratory carbon dioxide concentration is also used toestimate the inspiratory volume or the expiratory volume. The means forestimating the energy expenditure is preferably further operable toestimate the energy expenditure of the patient based on an estimate ofinspiratory volume or an estimate of expiratory volume.

The apparatus preferably further comprises means for correcting ameasurement in the set of measurements to produce a correctedmeasurement, wherein the corrected measurement compensates for adifference between a thermodynamic condition at the ventilator and arespective thermodynamic condition at the patient, and wherein the meansfor estimating the energy expenditure of the patient is further operableto estimate the energy expenditure based on the corrected measurement.By correcting a measurement in this manner, the accuracy of theestimated energy expenditure can be improved. The corrected measurementpreferably compensates for a difference in temperature and/or relativehumidity. The means for correcting a measurement is preferably operableto correct an expiratory volume measurement or an inspiratory volumemeasurement.

The set of measurements preferably further comprises a respiratoryfrequency. Respiratory frequency is another measurement that is alreadyavailable from many existing ventilators, and which can advantageouslybe used to estimate the daily energy expenditure of the patient.

The apparatus can be integrated with the ventilator. Alternatively, theapparatus can be detachably connectable to the ventilator. In the lattercase, the apparatus can be retrofitted to an existing ventilator, orsupplied as an optional add-on unit, to provide the additionalfunctionality of measuring the energy expenditure of a patient.

A further aspect of the invention provides a method for estimatingenergy expenditure of a patient, the method comprising: receiving a setof measurements from a ventilator, wherein the set of measurementscomprises at least one gas concentration measurement; and estimating theenergy expenditure of the patient based on the set of measurements. Theat least one gas concentration measurement preferably comprises anexpiratory carbon dioxide concentration measurement and/or aninspiratory oxygen concentration measurement.

The method preferably further comprises estimating an inspiratory carbondioxide concentration based on the inspiratory oxygen concentrationmeasurement. The method preferably further comprises estimating aninspiratory carbon dioxide concentration based on a known concentrationof carbon dioxide in medical air. The step of estimating the energyexpenditure is preferably based on an estimate of inspiratory carbondioxide concentration.

The method preferably further comprises receiving an expiratory oxygenconcentration measurement, and the step of estimating the energyexpenditure preferably comprises estimating the energy expenditure basedon the set of measurements and the expiratory oxygen concentrationmeasurement. The expiratory oxygen concentration measurement ispreferably received from a sensor that is separate from the ventilator.

The set of measurements preferably further comprises an expiratoryvolume measurement or an inspiratory volume measurement. The methodpreferably further comprises estimating an inspiratory volume or anexpiratory volume based on the set of measurements. The step ofestimating the energy expenditure is preferably based on an estimate ofinspiratory volume or an estimate of expiratory volume.

The method preferably further comprises: correcting a measurement in theset of measurements to produce a corrected measurement, wherein thecorrected measurement compensates for a difference between athermodynamic condition at the ventilator and a respective thermodynamiccondition at the patient, and wherein the step of estimating the energyexpenditure of the patient further comprises estimating the energyexpenditure based on the corrected measurement. The correctedmeasurement preferably compensates for a difference in temperatureand/or relative humidity. The set of measurements preferably furthercomprises an expiratory volume measurement, and the step of correcting ameasurement preferably comprises correcting the expiratory volumemeasurement. The set of measurements preferably further comprises arespiratory frequency. The method can be performed by the ventilatoritself, or by an apparatus that is detachably connected to theventilator.

A further aspect of the invention provides a processor-readable mediumcomprising instructions which, when executed by a processor, cause theprocessor to perform a method for estimating energy expenditure of apatient, the method comprising: receiving a set of measurements from aventilator, wherein the set of measurements comprises at least one gasconcentration measurement; and estimating the energy expenditure of thepatient based on the set of measurements. The processor-readable mediumcan be integrated with the ventilator.

A further aspect of the invention provides an apparatus substantially asdescribed herein and/or as illustrated in any of the accompanyingdrawings. A further aspect of the invention provides a methodsubstantially as described herein and/or as illustrated in any of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the invention will now be described, purely by wayof example, with reference to the accompanying drawings, wherein likeelements are indicated using like reference signs, and in which:

FIG. 1 is a schematic diagram illustrating an apparatus for estimatingthe energy expenditure of a patient in accordance with the presentinvention;

FIG. 2 is a schematic diagram illustrating thermodynamic conditions at apatient and a ventilator;

FIG. 3 is an example of a user interface for the apparatus shown in FIG.1;

FIG. 4 is a flow chart of a method in accordance with the presentinvention; and

FIG. 5 is a schematic diagram of a computer system that may be used toimplement a method in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an apparatus 1 for estimating the energy expenditureof a patient 2. The apparatus 1 is coupled to a ventilator 4. In use,the ventilator 4 is coupled to the patient 2 by pulmonary tubes 10,thereby to allow the patient 2 to receive an inspiratory gas 12 providedby the ventilator 4 and to allow the return of an expiratory gas 14 tothe ventilator 4. The pulmonary tubes 10 comprise a first pulmonary tube10 a to conduct the inspiratory gas 12 and a second pulmonary tube 10 bto conduct the expiratory gas 14. The first and second pulmonary tubes10 a, 10 b are connected by a joint 17 near the patient 2. The joint 17may comprise a T-piece or a Y-piece. A humidifier 22 may optionally beconnected to the first pulmonary tube 10 a to increase the humidity ofthe inspiratory gas 12.

The apparatus 1 includes a first input 3, a second input 5, a processingmeans 7 and a display 20. The first input 3 is arranged to receive a setof measurements 6 from the ventilator 4. The second input 5 is arrangedto receive a further set of measurements 8 from one or more sensors 16that are separate from the ventilator 4. The first input 3 and thesecond input 5 are coupled to a processing means 7. The processing means7 is operable to perform calculations, as described in more detailbelow, including calculations to estimate the energy expenditure of thepatient 2 based on the set of measurements 6. The processing means 7could comprise a personal computer, a microprocessor, a microcontroller,a digital signal processor, programmable logic, software, firmwareand/or any other means suitable for estimating the energy expenditure ofthe patient.

The display 20 is operable to present various measurements and estimatesto a user. In particular, the display 20 is operable to display theenergy expenditure estimated by the processing means 7. The display 20may also be operable to display any measurement selected from the set ofmeasurements 6 from the ventilator 4 and/or the further set ofmeasurements 8 from the one or more sensors 16. The display 20 may bethe monitor of a personal computer used to implement the processingmeans 7.

In the example shown in FIG. 1, the apparatus 1 is illustrated as beinga separate entity from the ventilator 4. In such an example, theapparatus 1 is detachably coupled to the ventilator 4. For example, theapparatus 1 may be implemented using a personal computer, which can becoupled to the ventilator 4 by a suitable communication interface. Thecommunication interface may be a wired communication interface (such asan Ethernet, serial port or universal serial bus (USB) interface) or awireless communication interface (such as an IEEE 802.11 (Wi-Fi®) orBluetooth® interface).

In other examples, the apparatus 1 can be integrated with the ventilator4. In these examples, the processing means 7 may comprise the processorthat is usually used by the ventilator 4 to monitor and control thepatient's breathing, whilst the display 20 may comprise the display thatis usually used by the ventilator 4 to display data relating to thepatient's breathing.

In use, the ventilator 4 measures various properties of the inspiratorygas 12 and the expiratory gas 14 in order to ensure that the patient isreceiving appropriate breathing assistance. Of the many measurementsthat are made by the ventilator 4, the following measurements are ofparticular relevance to the present invention:

-   -   expiratory volume (V_(e)), which is a measurement of the total        volume of air exhaled by the patient (typically measured in        millilitres);    -   inspiratory volume (V_(i)) which is a measurement of the total        volume of air inhaled by the patient (typically measured in        millilitres);    -   inspiratory oxygen concentration (F_(i) _(O2) ), which is a        measurement of the proportion of oxygen in the air inhaled by        the patient (expressed as a percentage);    -   expiratory carbon dioxide concentration (F_(e) _(CO2) ), which        is a measurement of the proportion of carbon dioxide in the air        exhaled by the patient (expressed as a percentage); and    -   breathing frequency (f), which is a measurement of the number of        breaths taken by the patient per unit time (typically measured        in breaths per minute).

These measurements are made by the ventilator 4 and provided to thefirst input 3 of the apparatus 1 as the set of measurements 6. Anexample of a ventilator 4 that is suitable for providing the set ofmeasurements 6 is the Dräger Evita® XL, manufactured by Drägerwerk AG &Co. Other suitable ventilators 4 could be used. In an example in whichthe ventilator 4 is a Dräger Evita® XL (or another ventilator withsimilar functionality), the apparatus 1 can be coupled to the ventilator4 by an RS-232 connection, and the ventilator 4 can send the set ofmeasurements 6 to the apparatus 1 using the LUST protocol. The LUSTprotocol is a proprietary protocol that is implemented in the DrägerEvita® XL, and is capable of sending four types of information:identification information; status information; data; and alarms. Inthis example, measurements of V_(e), F_(i) _(O2) and F_(e) _(CO2) areincluded in the identification information that is communicated from theventilator 4 to the apparatus 1. A measurement of the pressure at theend of the patient's exhalation can also be included in theidentification information that is communicated from the ventilator 4 tothe apparatus 1.

Whilst known ventilators can measure many different properties of theinspiratory gas 12 and the expiratory gas 14, they are not designed tomeasure the expiratory oxygen concentration or the inspiratory carbondioxide concentration because these measurements are not considered tobe useful for achieving the ventilator's primary purpose of ensuringthat the patient receives appropriate breathing assistance. Theexpiratory oxygen concentration (F_(e) _(O2) ) is a measurement of theproportion of oxygen in the air exhaled by the patient (expressed as apercentage). The inspiratory carbon dioxide concentration (F_(i) _(CO2)) is a measurement of the proportion of carbon dioxide in the airinhaled by the patient (expressed as a percentage).

Since known ventilators do not measure the expiratory oxygenconcentration, the sensors 16 preferably comprise an oxygenconcentration sensor 16 a for measuring the expiratory oxygenconcentration. Suitable oxygen concentration sensors are known in theart. Purely by way of example, the oxygen concentration sensor 16 a maybe an AX300-I Portable Oxygen Analyzer, manufactured by TeledyneAnalytical Instruments of California, USA. The second input 5 isarranged to receive an expiratory oxygen concentration measurement fromthe oxygen concentration sensor 16 a. In the example shown in FIG. 1,the oxygen concentration sensor 16 a is separate from the ventilator 4.

The oxygen concentration sensor 16 a is preferably located within thejoint 17. Locating the oxygen concentration sensor 16 a within the joint17 allows the expiratory oxygen concentration to be measured very closeto the patient 2, which means that the expiratory oxygen concentrationis measured under substantially the same thermodynamic conditions thatexist at the patient. This avoids the need to correct the expiratoryoxygen concentration measurement to compensate for differences inthermodynamic conditions between the patient and the point at which itis measured. For similar reasons, if the sensors 16 comprise any sensorsother than the oxygen concentration sensor 16 a, these are alsopreferably located within the joint 17.

Although ventilators that are currently on the market are not capable ofmeasuring expiratory oxygen concentration, future ventilators may bearranged to measure expiratory oxygen concentration. For example, futureventilators may comprise an oxygen concentration sensor for the specificpurpose of measuring expiratory oxygen concentration, or they may use anexisting sensor for measuring inspiratory oxygen concentration for thefurther purpose of measuring expiratory oxygen concentration. The firstinput 3 of the apparatus 1 would then receive the expiratory oxygenconcentration measurement from the ventilator 4. The present inventionpreferably encompasses apparatuses and methods for estimating energyexpenditure based on an expiratory oxygen concentration measurement thatis received from the ventilator 4.

The apparatus 1 could also be used with less-sophisticated ventilatorsthat are not necessarily able to measure each of the expiratory volume,inspiratory oxygen concentration, expiratory carbon dioxideconcentration and breathing frequency. When used with suchless-sophisticated ventilators, the one or more sensors 16 will compriseone or more additional sensors to measure the property that is notmeasured by the ventilator. However, at the very least, it is envisagedthat the apparatus 1 will be used with a ventilator 4 that is capable ofproviding at least one gas concentration measurement; the at least onegas concentration measurement could be any one or more of an inspiratoryoxygen concentration, an expiratory carbon dioxide concentration or evenan expiratory oxygen concentration, depending on the capabilities of theventilator 4.

For the sake of clarity, the following description will assume that theventilator 4 is capable of measuring the expiratory volume, theinspiratory oxygen concentration, expiratory carbon dioxideconcentration and breathing frequency, and that the expiratory oxygenconcentration measurement is received from a sensor 16 a that isseparate from the ventilator 4, although it will now be apparent thatthe invention is preferably not limited to this particular arrangement.

The processing means 7 is operable to estimate the energy expenditure ofthe patient 2 in the following manner. In order to estimate the energyexpenditure, it is necessary to know the oxygen elimination ({dot over(V)}_(O) ₂ ) and carbon dioxide production ({dot over (V)}_(CO) ₂ ),which may be expressed by the following equations:

{dot over (V)} _(CO) ₂ =V _(e) ×F _(e) _(CO2) −V _(i) ×F _(i) _(CO2)  (1)

{dot over (V)} _(O) ₂ =V _(i) ×F _(i) _(O2) −V _(e) ×F _(e) _(O2)   (2)

As explained above, V_(e) is the expiratory volume, V_(i) is theinspiratory volume, F_(i) _(CO2) is the inspiratory carbon dioxideconcentration, F_(e) _(CO2) is the expiratory carbon dioxideconcentration, F_(i) _(O2) is the inspiratory oxygen concentration andF_(e) _(O2) is the expiratory oxygen concentration.

Using the values for {dot over (V)}_(CO) ₂ and {dot over (V)}_(O) ₂calculated in accordance with equations (1) and (2) respectively, theenergy expenditure of the patient 2 is then calculated using the Weirformula (Weir, 1949):

EE=3.9×{dot over (V)} _(O) ₂ +1.1×{dot over (V)} _(CO) ₂   (3)

where EE is the energy expenditure measured in kilocalories per breath.

It can be seen from equations (1), (2) and (3) that calculation of thepatient's energy expenditure involves six variables, i.e. V_(i), V_(e),F_(i) _(CO2) , F_(e) _(CO2) , F_(i) _(O2) and F_(e) _(O2) . Theinventors have discovered that the patient's energy expenditure can beestimated reliably using measurements of V_(e), F_(e) _(CO2) and F_(i)_(O2) made by the ventilator 4, plus a measurement of F_(e) _(O2)provided by the oxygen concentration sensor 16 a. Thus, the need for aseparate indirect calorimeter can be avoided, and the number of sensorscan be reduced, by making use of measurements that are already availablefrom the ventilator 4.

As mentioned above, known ventilators do not measure the inspiratorycarbon dioxide concentration, F_(i) _(CO2) . Furthermore, theinspiratory carbon dioxide concentration is so small that it isdifficult to measure reliably. The inventors have discovered that thepatient's energy expenditure can be estimated reliably, withoutmeasuring the inspiratory carbon dioxide concentration, based upon anestimate of the inspiratory carbon dioxide concentration. By way ofexplanation, the inspiratory gas 12 usually comprises medical air mixedwith oxygen in a known ratio; that is, in addition to the oxygen that isalready present in the medical air, the inspiratory gas 12 comprises aknown amount of supplementary oxygen. The concentration of carbondioxide in medical air is known a priori. For example, the medical dryair that is commonly found in hospitals and provided to the patient 2 bythe ventilator 4 typically has a carbon dioxide concentration of 0.039%.Thus, it is possible to calculate the inspiratory carbon dioxideconcentration based upon an inspiratory oxygen concentrationmeasurement, the known ratio between medical air and supplementaryoxygen in the inspiratory gas 12, and the known concentration of carbondioxide in medial air. The processing means 7 is preferably operable tocalculate the inspiratory carbon dioxide concentration as a function ofthe inspiratory oxygen concentration that is measured by the ventilator4. For example, when the carbon dioxide concentration is assumed to be0.039%, the inspiratory carbon dioxide concentration can be calculatedusing the following equation:

F_(i) _(CO2)=0.039×(120.95−F _(i) _(O2) )   (4)

The resulting estimate of the inspiratory carbon dioxide concentrationcan be used as the value for F_(i) _(CO2) in equation (1). Thisadvantageously avoids the need for a sensor to measure the inspiratorycarbon dioxide concentration. Furthermore, this also avoids the error inthe estimate of the patient's energy expenditure that would result fromthe inherent inaccuracy of measuring the inspiratory carbon dioxideconcentration directly.

Whilst some ventilators (such as the Dräger Evita® XL) are capable ofmeasuring the inspiratory volume, V_(i), it is preferable not to use ameasurement of the inspiratory volume to estimate the patient's energyexpenditure. This is because the presence of errors in the measurementsof both expiratory volume and inspiratory volume would result in a largeerror in the estimated energy expenditure. The inventors have discoveredthat the patient's energy expenditure can be estimated more accurately,without measuring the inspiratory volume, based upon an estimate of theinspiratory volume. Hence, the processing means 7 is preferably operableto calculate the inspiratory volume as a function of the measuredexpiratory volume, the measured expiratory oxygen concentration, themeasured expiratory carbon dioxide concentration, the measuredinspiratory oxygen concentration and the estimated inspiratory carbondioxide concentration. The inspiratory volume can be calculated usingthe Haldane transformation:

$\begin{matrix}{V_{i} = \frac{V_{e}\left( {1 - F_{e_{O\; 2}} - F_{e_{{CO}\; 2}}} \right)}{1 - F_{i_{O\; 2}} - F_{i_{{CO}\; 2}}}} & (5)\end{matrix}$

Calculating the inspiratory volume as a function of the measuredexpiratory volume in this manner has the effect of correlating the errorin the inspiratory and expiratory volumes, which reduces the error inthe estimate of the energy expenditure of the patient.

Alternatively, the patient's energy expenditure can be estimated basedupon an estimate of the expiratory volume. In this case, the inspiratoryvolume is measured by the ventilator 4, and the processing means 7 isoperable to calculate the expiratory volume as a function of themeasured inspiratory volume, the measured expiratory oxygenconcentration, the measured expiratory carbon dioxide concentration, themeasured inspiratory oxygen concentration and the estimated inspiratorycarbon dioxide concentration. The expiratory volume can be calculated byrearranging equation (5) to yield:

$\begin{matrix}{V_{e} = \frac{V_{i}\left( {1 - F_{i_{O\; 2}} - F_{i_{{CO}\; 2}}} \right)}{1 - F_{e_{O\; 2}} - F_{e_{{CO}\; 2}}}} & \left( 5^{\prime} \right)\end{matrix}$

Calculating the expiratory volume as a function of the measuredinspiratory volume in this manner also has the effect of correlating theerror in the inspiratory and expiratory volumes, which reduces the errorin the estimated energy expenditure.

The processing means 7 is operable to estimated the energy expenditureof the patient 2 based on a set of measurements 6 received from theventilator 4 and equations (1) to (5). The set of measurements 6comprises at least one gas concentration measurement. The set ofmeasurements preferably comprises an expiratory volume measurement (or,alternatively, an inspiratory volume measurement), an inspiratory oxygenconcentration measurement and an expiratory carbon dioxide concentrationmeasurement. The energy expenditure that is estimated by the processingmeans 7 is preferably also based on an expiratory oxygen concentrationmeasurement, which may be received from an external sensor 16 a or fromthe ventilator 4.

A new estimate of the patient's energy expenditure can be calculatedusing equation (3) each time that the patient breathes. To achieve this,the processing means 7 can monitor the expiratory volume, V_(e), todetect changes indicative of the patient exhaling. Upon detecting thatthe patient has exhaled, new sets of measurements 6, 8 can be taken, anda new estimate of energy expenditure can be calculated using equation(3).

As mentioned above, equation (3) allows energy expenditure to becalculated in kilocalories per breath. The processing means 7 can alsocalculate the energy expenditure in kilocalories per day using thebreathing frequency (f) measured by the ventilator 4. For a ventilator 4that measures the breathing frequency in breaths per minute, the energyexpenditure in kilocalories per day (EE′) can be calculated using theenergy expenditure in kilocalories per breath (EE) and the followingequation:

EE′=EE×f×60×24   (6)

The energy expenditure in kilocalories per day (EE′) is clinically moreuseful than the energy expenditure in kilocalories per breath (EE)because it allows the correct amount of nutrition required by thepatient to be directly determined. When determining how much nutritionto give to the patient, the average (mean) value of a large number ofestimates of energy expenditure in kilocalories per day is preferablyused, each estimate of energy expenditure in kilocalories per day beingcalculated based upon a respective estimate of energy expenditure inkilocalories per breath.

The fundamental principles of estimating the energy expenditure of apatient in accordance with the present invention have thus beendescribed. This approach can optionally be improved by performingcorrections upon the set of measurements 6 received from the ventilator4. The purpose of these corrections is to account for the fact that theventilator 4 takes its measurements at different thermodynamicconditions from those that exist at the lungs of the patient 2. Threemain factors can cause a divergence between the set of measurements 6and the corresponding properties of the gas that is actually deliveredto the lungs:

-   -   the temperature difference between the ventilator 4 and the        mouth of the patient 2 as gas travels through the pulmonary        tubes 10;    -   if there is a humidifier 22 connected between the ventilator 4        and the patient 2, the water vapour generated by the humidifier        will alter the concentrations of oxygen and carbon dioxide, as        well as the pressure and temperature; and    -   the compliance and resistance of the pulmonary tubes 10, which        will alter the inspiratory volume delivered from the ventilator        4.

These factors can be corrected for using models such as: the ideal gaslaw; Dalton's law for adding partial pressures; and calculating theinfluence of compliance and resistance of the pulmonary tubes on thetidal volume.

FIG. 2 illustrates the different thermodynamic conditions that existwhen a humidifier 22 is connected between a ventilator 4 and a patient2. In FIG. 2, T denotes temperature, V denotes volume, the subscript idenotes a property of the inspiratory gas, the subscript e denotes aproperty of the expiratory gas, the subscript patient denotes a propertymeasured at the patient 2, and the subscript MV denotes a propertymeasured at the ventilator 4. Thus, FIG. 2 shows that the temperaturesand volumes of the inspiratory and expiratory gases differ dependingupon whether they are measured at the ventilator 4 or at the patient 2.FIG. 2 also shows that the relative humidity of the inspiratory gas atthe outlet of the ventilator 4 is considered to be zero, whereas therelative humidity of the inspiratory gas at the outlet of the humidifier22 is considered to be 100%. The relative humidity of the expiratory gasis considered to be 100% at both the patient 2 and the ventilator 4.

A method of correcting the expiratory volume (V_(e)) measurement toaccount for the temperature difference between the ventilator 4 and themouth of the patient 2, and also to account for the presence of ahumidifier 22, will now be described.

The expiratory volume measurement that the ventilator 4 provides to theapparatus 1 as part of the set of measurements 6 is expressed in BodyTemperature Pressure Saturated (BTPS) conditions. Measurements expressedin BTPS conditions assume that a gas has 100% relative humidity, atemperature of 37° C. and a pressure of 101.325 kPa. In order to expressthe expiratory volume measurement in BTPS conditions, the ventilator 4automatically converts the measured expiratory volume that is actuallymeasured to BTPS conditions, assuming that the expiratory volume wasmeasured at a temperature of T_(y)° C. and a relative humidity of y %.For example, the Dräger Evita® XL assumes that T_(y) is 30° C. and thaty is 100%. However, Equations (1), (2) and (3) assume that theexpiratory volume is expressed in Normal Temperature Pressure Dry (NTPD)conditions. Measurements expressed in NTPD conditions assume that a gashas 0% relative humidity, a temperature of 20° C. and a pressure of101.325 kPa. Thus, the processing means 7 preferably converts theexpiratory volume measurement received from the ventilator 4 to NTPDconditions, taking into account that it was not measured at the assumedconditions of T_(y)° C. and y % relative humidity, but was actuallymeasured at a temperature of T_(x)° C. and a relative humidity of x %.The processing means 7 performs this calculation using the followingequation:

$\begin{matrix}{V_{e} = {{{V_{e_{MV}} \cdot \frac{\left( {T_{y} + 273.2} \right) \cdot (273.2)}{310.2} \cdot \frac{P - {P_{100\% \mspace{11mu} {RH}}\left( {37{^\circ}\mspace{14mu} {C.}} \right)}}{P\left( {P - {P_{100\% \mspace{11mu} {RH}}\left( {T_{y}^{{^\circ}}\mspace{14mu} {C.}} \right)}} \right)}}P} - {P_{x\mspace{11mu} \% \mspace{11mu} {RH}}\left( {T_{x}^{{^\circ}}\mspace{14mu} {C.}} \right)}}} & (7)\end{matrix}$

where V_(e) is the corrected expiratory volume measurement at NTPDconditions, V_(e) _(MV) is the expiratory volume measurement in BTPSconditions that is provided by the ventilator 4 to the apparatus 1, P isambient atmospheric pressure (i.e. 101.325 kPa for NTPD conditions), andP_(a % RH)(b° C.) denotes the partial pressure of water vapour at a %relative humidity and a temperature of b° C. The temperature T_(x) willdepend on the temperature setting of the humidifier 22. T_(x) can bedetermined empirically, by measuring the temperature when calibratingthe apparatus 1. For example, the sensors 16 can include a temperaturesensor (not shown in FIG. 1) for measuring temperature. The temperaturesensor may comprise a thermocouple. Purely by way of example, a suitabletemperature sensor is a J-type exposed-junction thermocouplemanufactured by National Instruments Corporation. The relative humidityx % can also be determined empirically, by measuring relative humiditywhen calibrating the apparatus 1. For example, the sensors 16 caninclude a humidity sensor (not shown in FIG. 1) for measuring humidity.Purely by way of example, a suitable humidity sensor is a HIH-4000integrated circuit humidity sensor, manufactured by Honeywell.Alternatively, the relative humidity x % can be assumed to have a valuebetween 90% and 100%, and preferably a value of 95%.

The corrected expiratory volume measurement at NTPD conditions (V_(e))given by Equation (7) can be substituted into Equations (1) and (2),such that Equation (3) yields a more accurate estimate of the patient'senergy expenditure.

In the alternative example described above, in which the inspiratoryvolume (rather than the expiratory volume) is measured by the ventilator4, the inspiratory volume can be corrected in a similar manner. In thiscase, the patient's energy expenditure can be estimated based upon thecorrected inspiratory volume measurement.

Whilst performing such corrections can improve the accuracy of theestimate of a patient's energy expenditure, there will always be factorsaffecting the estimate that cannot be identified or controlled. Todemonstrate the effectiveness of the method and apparatus that isdisclosed herein, the inventors have analysed the impact of systematicerrors and random errors upon the energy expenditure estimate. Themagnitude of systematic errors was estimated comparing each measurementin the set of measurements 6 taken by a Dräger Evita® XL ventilator witha corresponding measurement taken by an external measuring device. Themagnitude of random errors was calculated from the standard deviationsin repeated measurements of the same property with the ventilator. Thetotal systematic error was found to be 8.3%, whilst the total randomerror was found to be 0.5%. Assuming that the systematic error andrandom error are uncorrelated, the total error is equal to the squareroot of the sum of the squares of the systematic and random errors.Thus, the total error was found to be 8.3%, i.e. (0.083²+0.005²)^(0.5).This total error is sufficiently small for the method and apparatus thatare disclosed herein to be suitable for clinical use.

FIG. 3 is an example of a user interface 300 for an apparatus forestimating the energy expenditure of a patient. The user interface 300can be presented on the display 20 of the apparatus 1. The userinterface 300 comprises a plurality of regions 302, 304, 306, 308.Region 302 is operable to display one or more measurements made by theventilator 4 and/or the sensors 16. Region 304 is operable to displayone or more values that are calculated based upon measurements made bythe ventilator 4 and/or the sensors 16. Region 306 is operable toreceive a user input to specify the values of parameters used toestimate the energy expenditure of a patient. For example, region 306allows a user to specify any one or more of the following parameters:ambient atmospheric pressure; a Boolean value indicating whether ahumidifier 22 is operating; and a Boolean value indicating whether thepatient 2 is receiving breathing assistance via face mask or a trachealtube. Region 308 is operable to display the energy expenditure 310 ofthe patient 2.

FIG. 4 is a flow diagram of a method 100 for estimating the energyexpenditure of a patient 2. In step 102, a set of measurements 6 isreceived from the ventilator 4. As mentioned previously, the set ofmeasurements 6 preferably comprises an expiratory volume measurement, aninspiratory oxygen concentration measurement and an expiratory carbondioxide concentration measurement. In step 104, a measurement 8 isreceived from an external sensor 16. The measurement 8 that is receivedfrom the external sensor 16 is preferably an expiratory oxygenconcentration measurement. Whilst FIG. 4 shows that step 104 precedesstep 102, it will be appreciated that steps 102 and 104 can be performedin any order or be performed simultaneously. In step 106, theinspiratory carbon dioxide concentration is estimated. In step 108, theinspiratory volume is estimated. In step 110, one or more of themeasurements in the set of measurements 6 from the ventilator 4 iscorrected to produce a respective corrected measurement. The correctedmeasurement compensates for a difference in thermodynamic conditionsexisting at the ventilator 4 and the patient 2. Whilst FIG. 4 shows thatsteps 106 and 108 precede step 110, it will be appreciated that thecorrected measurement may alternatively be performed before theinspiratory carbon dioxide concentration and inspiratory volume areestimated. In step 112, the energy expenditure of the patient 2 isestimated based upon at least the set of measurements 6 from theventilator 4. The energy expenditure can also be based upon theexpiratory oxygen concentration measurement, the estimated inspiratorycarbon dioxide concentration, the estimated inspiratory volume (orestimated expiratory volume) and/or a corrected measurement.

The method 100 can be implemented by a computer system 600 such as thatshown in FIG. 5. The invention can also be implemented as program codefor execution by the computer system 600. After reading thisdescription, it will become apparent to a person skilled in the art howto implement the invention using other computer systems and/or computerarchitectures.

Computer system 600 includes one or more processors, such as processor604. Processor 604 may be any type of processor, including but notlimited to a special purpose or a general-purpose digital signalprocessor. Processor 604 is connected to a communication infrastructure606 (for example, a bus or network). Computer system 600 also includes amain memory 608, preferably random access memory (RAM), and may alsoinclude a secondary memory 610. Secondary memory 610 may include, forexample, a hard disk drive 612 and/or a removable storage drive 614,representing a floppy disk drive, a magnetic tape drive, an optical diskdrive, etc. Removable storage drive 614 reads from and/or writes to aremovable storage unit 618 in a well-known manner. Removable storageunit 618 represents a floppy disk, magnetic tape, optical disk, etc.,which is read by and written to by removable storage drive 614. As willbe appreciated, removable storage unit 618 includes a computer usablestorage medium having stored therein computer software and/or data.

In alternative implementations, secondary memory 610 may include othersimilar means for allowing computer programs or other instructions to beloaded into computer system 600. Such means may include, for example, aremovable storage unit 622 and an interface 620. Examples of such meansmay include a program cartridge and cartridge interface (such as thatpreviously found in video game devices), a removable memory chip (suchas an EPROM, or PROM, or flash memory) and associated socket, and otherremovable storage units 622 and interfaces 620 which allow software anddata to be transferred from removable storage unit 622 to computersystem 600. Alternatively, the program may be executed and/or the dataaccessed from the removable storage unit 622, using the processor 604 ofthe computer system 600.

Computer system 600 may also include a communication interface 624.Communication interface 624 allows software and data to be transferredbetween computer system 600 and external devices. Examples ofcommunication interface 624 may include a modem, a network interface(such as an Ethernet card), a communication port etc. Software and datatransferred via communication interface 624 are in the form of signals628, which may be electronic, electromagnetic, optical, or other signalscapable of being received by communication interface 624. These signals628 are provided to communication interface 624 via a communication path626. Communication path 626 carries signals 628 and may be implementedusing wire or cable, fibre optics, a phone line, a wireless link, acellular phone link, a radio frequency link, or any other suitablecommunication channel. For instance, communication path 626 may beimplemented using a combination of channels.

The terms “computer program medium” and “computer usable medium” areused generally to refer to media such as removable storage drive 614, ahard disk installed in hard disk drive 612, and signals 628. Thesecomputer program products are means for providing software to computersystem 600. However, these terms may also include signals (such aselectrical, optical or electromagnetic signals) that embody the computerprogram disclosed herein.

Computer programs (also called computer control logic) are stored inmain memory 608 and/or secondary memory 610. Computer programs may alsobe received via communication interface 624. Such computer programs,when executed, enable computer system 600 to implement the methoddescribed herein. Accordingly, such computer programs representcontrollers of computer system 600. Where the method is implementedusing software, the software may be stored in a computer program productand loaded into computer system 600 using removable storage drive 614,hard disk drive 612, or communication interface 624, to provide someexamples.

In alternative embodiments, the invention can be implemented as controllogic in hardware, firmware, software or any combination thereof Theapparatus may be implemented by dedicated hardware, such as one or moreapplication-specific integrated circuits (ASICs) or appropriatelyconnected discrete logic gates. A suitable hardware description languagecan be used to implement the method described herein with dedicatedhardware.

The method 100 can be performed by instructions stored on aprocessor-readable medium. The processor-readable medium may be: aread-only memory (including a PROM, EPROM or EEPROM); random accessmemory; a flash memory; an electrical, electromagnetic or opticalsignal; a magnetic, optical or magneto-optical storage medium; one ormore registers of a processor; or any other type of processor-readablemedium.

It will be understood that the invention has been described above purelyby way of example, and that modifications of detail can be made withinthe scope of invention.

1. An apparatus for estimating energy expenditure of a patient, theapparatus comprising: means for receiving a set of measurements from aventilator, the ventilator being configured to measure at least one gasconcentration measurement in order to provide breathing assistance tothe patient, wherein the set of measurements comprises said at least onegas concentration measurement; means for estimating the energyexpenditure of the patient based on the set of measurements.
 2. Anapparatus in accordance with claim 1, wherein the at least one gasconcentration measurement comprises an expiratory carbon dioxideconcentration measurement.
 3. An apparatus in accordance with claim 1,wherein the at least one gas concentration measurement comprises aninspiratory oxygen concentration measurement.
 4. An apparatus inaccordance with claim 3, wherein the apparatus further comprises meansfor estimating an inspiratory carbon dioxide concentration based on theinspiratory oxygen concentration measurement.
 5. An apparatus inaccordance with claim 1, wherein the apparatus further comprises meansfor estimating an inspiratory carbon dioxide concentration based on aknown concentration of carbon dioxide in medical air.
 6. An apparatus inaccordance with claim 1, wherein the means for estimating the energyexpenditure is operable to estimate the energy expenditure based on anestimate of inspiratory carbon dioxide concentration.
 7. An apparatus inaccordance with claim 1, wherein the apparatus further comprises meansfor receiving an expiratory oxygen concentration measurement, andwherein the means for estimating the energy expenditure is operable toestimate the energy expenditure based on the set of measurements and theexpiratory oxygen concentration measurement.
 8. An apparatus inaccordance with claim 7, wherein the apparatus is operable to receivethe expiratory oxygen concentration measurement from a sensor that isseparate from the ventilator.
 9. An apparatus in accordance with claim1, wherein the set of measurements further comprises an expiratoryvolume measurement or an inspiratory volume measurement.
 10. Anapparatus in accordance with claim 1, wherein the apparatus furthercomprises means for estimating an inspiratory volume or an expiratoryvolume based on the set of measurements.
 11. An apparatus in accordancewith claim 1, wherein the means for estimating the energy expenditure isfurther operable to estimate the energy expenditure of the patient basedon an estimate of inspiratory volume or an estimate of expiratoryvolume.
 12. An apparatus in accordance with claim 1, wherein theapparatus further comprises means for correcting a measurement in theset of measurements to produce a corrected measurement, wherein thecorrected measurement compensates for a difference between athermodynamic condition at the ventilator and a respective thermodynamiccondition at the patient, and wherein the means for estimating theenergy expenditure of the patient is further operable to estimate theenergy expenditure based on the corrected measurement.
 13. An apparatusin accordance with claim 12, wherein the corrected measurementcompensates for a difference in temperature and/or relative humidity.14. An apparatus in accordance with claim 12, wherein the set ofmeasurements further comprises an expiratory volume measurement, andwherein the means for correcting a measurement is operable to correctthe expiratory volume measurement.
 15. An apparatus in accordance withclaim 1, wherein the set of measurements further comprises a respiratoryfrequency.
 16. An apparatus in accordance with claim 1, wherein theapparatus is integrated with the ventilator.
 17. An apparatus inaccordance with claim 1, wherein the apparatus is detachably connectableto the ventilator.
 18. A method for estimating energy expenditure of apatient, the method comprising: receiving a set of measurements from aventilator, the ventilator being configured to measure at least one gasconcentration measurement in order to provide breathing assistance tothe patient, wherein the set of measurements comprises said at least onegas concentration measurement; and estimating the energy expenditure ofthe patient based on the set of measurements.
 19. A method in accordancewith claim 18, wherein the at least one gas concentration measurementcomprises an expiratory carbon dioxide concentration measurement.
 20. Amethod in accordance with claim 18, wherein the at least one gasconcentration measurement comprises an inspiratory oxygen concentrationmeasurement.
 21. A method in accordance with claim 20, wherein themethod further comprises estimating an inspiratory carbon dioxideconcentration based on the inspiratory oxygen concentration measurement.22. A method in accordance with claim 18, wherein the method furthercomprises estimating an inspiratory carbon dioxide concentration basedon a known concentration of carbon dioxide in medical air.
 23. A methodin accordance with claim 18, wherein the step of estimating the energyexpenditure is based on an estimate of inspiratory carbon dioxideconcentration.
 24. A method in accordance with claim 18, wherein themethod further comprises receiving an expiratory oxygen concentrationmeasurement, and wherein the step of estimating the energy expenditurecomprises estimating the energy expenditure based on the set ofmeasurements and the expiratory oxygen concentration measurement.
 25. Amethod accordance with claim 24, wherein the expiratory oxygenconcentration measurement is received from a sensor that is separatefrom the ventilator.
 26. A method in accordance with claim 18, whereinthe set of measurements further comprises an expiratory volumemeasurement or an inspiratory volume measurement.
 27. A method inaccordance with claim 18, wherein the method further comprisesestimating an inspiratory volume or an expiratory volume based on theset of measurements.
 28. A method in accordance with claim 18, whereinthe step of estimating the energy expenditure is based on an estimate ofinspiratory volume or an estimate of expiratory volume.
 29. A method inaccordance with claim 18, wherein the method further comprisescorrecting a measurement in the set of measurements to produce acorrected measurement, wherein the corrected measurement compensates fora difference between a thermodynamic condition at the ventilator and arespective thermodynamic condition at the patient, and wherein the stepof estimating the energy expenditure of the patient further comprisesestimating the energy expenditure based on the corrected measurement.30. A method in accordance with claim 29, wherein the correctedmeasurement compensates for a difference in temperature and/or relativehumidity.
 31. A method in accordance with claim 29, wherein the set ofmeasurements further comprises an expiratory volume measurement, andwherein the step of correcting a measurement comprises correcting theexpiratory volume measurement.
 32. A method in accordance with claim 18,wherein the set of measurements further comprises a respiratoryfrequency.
 33. A processor-readable medium comprising instructionswhich, when executed by a processor, cause the processor to perform amethod for estimating energy expenditure of a patient, the methodcomprising: receiving a set of measurements from a ventilator, theventilator being configured to measure at least one gas concentrationmeasurement in order to provide breathing assistance to the patient,wherein the set of measurements comprises said at least one gasconcentration measurement; and estimating the energy expenditure of thepatient based on the set of measurements.
 34. A processor-readablemedium in accordance with claim 33, wherein the at least one gasconcentration measurement comprises an expiratory carbon dioxideconcentration measurement.
 35. A processor-readable medium in accordancewith claim 33, wherein the at least one gas concentration measurementcomprises an inspiratory oxygen concentration measurement.
 36. Aprocessor-readable medium in accordance with claim 35, wherein themethod further comprises estimating an inspiratory carbon dioxideconcentration based on the inspiratory oxygen concentration measurement.37. A processor-readable medium in accordance with claim 33, wherein themethod further comprises estimating an inspiratory carbon dioxideconcentration based on a known concentration of carbon dioxide inmedical air.
 38. A processor-readable medium in accordance with claim33, wherein the step of estimating the energy expenditure is based on anestimate of inspiratory carbon dioxide concentration.
 39. Aprocessor-readable medium in accordance with claim 33, wherein themethod further comprises receiving an expiratory oxygen concentrationmeasurement, and wherein the step of estimating the energy expenditurecomprises estimating the energy expenditure based on the set ofmeasurements and the expiratory oxygen concentration measurement.
 40. Aprocessor-readable medium accordance with claim 39, wherein theexpiratory oxygen concentration measurement is received from a sensorthat is separate from the ventilator.
 41. A processor-readable medium inaccordance with claim 33, wherein the set of measurements furthercomprises an expiratory volume measurement or an inspiratory volumemeasurement.
 42. A processor-readable medium in accordance with claim41, wherein the method further comprises estimating an inspiratoryvolume or an expiratory volume based on the set of measurements.
 43. Aprocessor-readable medium in accordance with claim 42, wherein the stepof estimating the energy expenditure is based on an estimate ofinspiratory volume or an estimate of expiratory volume.
 44. Aprocessor-readable medium in accordance with claim 33, wherein themethod further comprises correcting a measurement in the set ofmeasurements to produce a corrected measurement, wherein the correctedmeasurement compensates for a difference between a thermodynamiccondition at the ventilator and a respective thermodynamic condition atthe patient, and wherein the step of estimating the energy expenditureof the patient further comprises estimating the energy expenditure basedon the corrected measurement.
 45. A processor-readable medium inaccordance with claim 44, wherein the corrected measurement compensatesfor a difference in temperature and/or relative humidity.
 46. Aprocessor-readable medium in accordance with claim 44, wherein the setof measurements further comprises an expiratory volume measurement, andwherein the step of correcting a measurement comprises correcting theexpiratory volume measurement.
 47. A processor-readable medium inaccordance with claim 33, wherein the set of measurements furthercomprises a respiratory frequency.
 48. A processor-readable medium inaccordance with claim 33, wherein the processor-readable medium isintegrated with the ventilator.